As processes for forming electric circuits (wiring conductors) of printed wiring boards, electronic parts and such, there has generally been known a method which comprises coating or printing an electroconductive paste containing silver powder with excellent conductivity as described on pages 42-46 of the October 1994 issue of Denshi Zairyo (Electronic Materials).
Due to their good electroconductivity, the electroconductive pastes containing silver powder have been used for forming electric circuits and electrodes of the printed wiring boards, electronic parts and such. However, the volume resistivity (specific resistance) of the electric circuits formed by using these electroconductive pastes usually fall in the range of 50 to 100 .mu..OMEGA..multidot.cm, and may reach only 30-40 .mu..OMEGA..multidot.cm at best, so that although it may cause few problems in case the length of the printed circuits was small, such as several cm or less, troubles were apt to arise when the circuit length was made 10 cm or longer due to an increase of conductor resistance.
For obtaining a conductor with good conductor resistance, the content of silver power is increased, but it is hardly possible to obtain stably a specific resistance, or resistivity, of 25 .mu..OMEGA..multidot.cm or less by this means. Simple increase of the silver powder content gives rise to problems such as upset balance with other properties such as adhesiveness.
The method comprising etching of a metal foil such as silver or copper foil can provide a high electroconductivity and a low resistivity on the order of several .mu..OMEGA..multidot.cm, but this method has a disadvantage in that it elevates the production cost as the process is complicated. Also, the electroconductive pastes using silver powder have a defect in that when an electric field is applied under a high-temperature and high-humidity atmosphere, there takes place a phenomenon called migration and resulting electrodeposition of silver between the wiring conductors and electrodes to cause short circuiting between the electrodes or the wires. For preventing such migration, several measures have been taken or are under study, such as coating the conductor surfaces with a moisture proof coating material or adding a corrosion inhibitor such as a nitrogen-containing compound to the electroconductive paste, but no satisfactory effect has ever been obtained with these measures.
It is also suggested to use silver-palladium alloy powder in place of silver powder for preventing migration. However, such alloy powder is costly as compared with silver powder, and although it is practically used for the small-sized wiring boards, such as those for hybrid IC, but it has not yet been practically applied to the substrates of the large-sized wiring boards, such as paper phenol substrate, glass epoxy substrate and polyethylene terephthalate substrate. Use of silver-coated copper powder can improve the migration problem and makes it possible to obtain an inexpensive electroconductive paste, but when silver coating is applied uniformly and thickly, no migration improving effect is produced. Plating is an economical way for coating; for instance silver plating on an inexpensive spherical copper powder can be easily performed with minimized possibility of flocculation, but this method has a disadvantage in that the electroconductive paste using such silver-plated powder is increased in resistance.
An object of the present invention, therefore, is to provide a composite electroconductive powder which is capable of producing an electroconductive paste for forming electric circuits which is low in resistivity, high in electroconductivity and minimized in change of resistivity even after a thermal shock test and/or a humidity and DC applied test.
Another object of the present invention is to provide a composite electroconductive powder capable of producing an electroconductive paste which is particularly excellent in electroconductivity and also excellent in oxidation resistance and heat resistance.
Still another object of the present invention is to provide a composite electroconductive powder capable of producing an electroconductive paste having an anchoring effect when pressed.
Yet another object of the present invention is to provide a composite electroconductive powder capable of producing an electroconductive paste for forming electric circuits which is low in resistivity, high in conductivity and excellent in migration resistance.
A further object of the present invention is to provide a composite electroconductive powder capable of producing an electroconductive paste which is low in resistivity, high in conductivity, minimized in change of resistivity even after a thermal shock test and a humidity and DC applied test, and capable of improving the probability of contact between the electroconductive powder particles, elevating electroconductivity of the electric circuits and also boosting electroconductivity particularly when a circuit is printed on a sheet-like substrate and the printed circuit is pressed.
Another object of the present invention is to provide an electroconductive paste for forming electric circuits which is low in resistivity, high in electroconductivity and minimized in change of resistivity even after a thermal shock test and/or a humidity and DC applied test.
Still another object of the present invention is to provide an electroconductive paste for forming electric circuits which is low in resistivity, capable of improving the probability of contact between the electroconductive powder particles and elevating electroconductivity of the electric circuits, and also excellent in migration resistance.
Still another object of the present invention is to provide an electric circuit which is low in resistivity, high in electroconductivity and excellent in migration resistance.
Yet another object of the present invention is to provide an electric circuit suited for forming fine circuits.
A further object of the present invention is to provide a process for producing an electric circuit which is low in resistivity, high in electroconductivity and excellent in migration resistance.
An additional object of the present invention is to provide a process for producing an electric circuit suited for forming fine circuits.